Prints made from images captured on digital cameras can have
extraordinary
tonal quality, comparable to the finest full-toned traditional prints.
But to reach their full potential they must be processed properly,
which
involves changing default camera settings and moving away from standard
file formats. This page covers the key steps for achieving ultimate
tonal
quality, which include,

Storing images in RAW format-- the image sensor's native format,

Converting and editing images using file formats with a bit depth
of 16
(48-bit color or 16-bit B&W), and

Using curves to adjust tones.

When you store images in RAW format you can also take advantage your
camera's
hidden
dynamic range; you can access tones that may be obscured
or lost when images are stored in a standard file format such as a
JPEG--
the default for most digital cameras.

JPEG is a lossy compressed format that sacrifices a small
amount
of information to achieve a large savings in storage. But it is not
this
information loss that concerns us here; it is the loss of tonal
levels that takes place when the image sensor's digital output
is
converted to any standard 24-bit
color
file format; JPEG is merely the most common. Image sensors in high
quality
digital cameras have a bit depth of at least 12; they have 212
= 4096 discrete tonal levels. Standard 24-bit color files have a bit
depth
of only 8; they have only 28 = 256 discrete tonal levels. Tonal
levels are lost in the conversion.
The techniques presented here minimize this loss-- they maximize tonal
detail throughout the RAW conversion and image editing process.

This
page
shows you how to obtain optimum tonal quality from your digital camera
and how to take advantage of its hidden dynamic range.

Introduction:
RAW conversion

Digital sensors (both CCD and CMOS) are
linear.
That means the voltage generated in each pixel, and hence the pixel
level
emerging from the A-to-D converter (the device that converts the sensor
output to discrete bits), is proportional to exposure-- to the light
energy
reaching the pixel. But neither human vision nor CRT monitors are
linear.
Hence the color spaces
(rules that map pixel levels to visible colors) used for standard image
files are intentionally nonlinear. The luminance represented by a pixel
is not proportional to the pixel level. Luminance in a
print
or monitor is related to pixel level by the following simplified
equation,

Output
luminance
= (pixel level)gamma

To obtain the pixels, the raw output of the image
sensor, which is proportional to exposure, must be converted to a file
with a standard color space using the inverse of the above operation.

pixel
level
= (RAW pixel level)1/gamma ~= exposure1/gamma

Gamma
is the exponent of the equation that relates luminance to pixel level.
Every color space has a characteristic gamma. Gamma = 2.2 for sRGB,
which is the default color space for Windows, the World Wide Web, and
most
digital cameras. sRGB has a color gamut that approximates typical CRT
monitors.
Another popular color space is Adobe RGB (1998), which has a slightly
larger
color gamut. It also has gamma = 2.2. ( Older Macintosh computers have
a default gamma of 1.8, but newer models seem to have switched to 2.2.)
In the
illustration
below, the first process-- converting light energy (RAW data) to file
pixels--
is shown on the left. The second process-- converting file pixels to
print
or display-- is shown on the right. The two processes are
complimentary;
when you combine them you get a straight line.

Figure 1. Characteristic curves for
the two
conversion processes in digital printmaking.

Output luminance (print or
monitor) as a function of normalized file pixel levels.

Both
shown with linear scales. The same
data is displayed
with logarithmic scales in Fig. 3.

Gamma correction is one of several functions
of
RAW
conversion-- the process of
converting
the output of an image sensor to a standard file format, such as JPEG
or
TIFF. Depending on your camera's Storage or Quality setting, RAW
conversion
takes place inside the camera, immediately after the exposure, or
afterwards
on a computer.

It should be evident from these graphs that
RAW
conversion compresses pixel levels representing high luminance and
expands
pixel levels representing low luminance. This means that the converted
file, with gamma = 2.2, has relatively fewer pixel levels in the
highlights
and more in the shadows. This turns out to be an advantage when human
vision
is considered.

RAW conversion performs several additional
functions.

Bayer
array
interpolation (de-mosaicing) is
the
most important function. Virtually all image sensors (except for the
Foveon
sensors used in the Sigma
SD9/SD10) use the Bayer
array pattern, where alternate rows of pixels represent RGRGRG...
and
GBGBGB..., but each pixel in standard image files (JPEG, TIFF,
etc.)
represents all three colors. RAW data is converted using a
sophisticated
process called interpolation, where data from the green sensors
(twice as plentiful as red and blue) is used to enhance the resolution
of the red and blue channels. The best Bayer interpolation routines--
the
routines that result in the highest resolution and the fewest
artifacts--
use iterative calculations, which are too slow to run on
digital
cameras, but present no problem for modern computers. See the section
on Bayer
sensors in Digital vs. film for
links
to technical papers.

Sharpening.
Unsharpened digital camera images tend to look soft; they benefit
strongly
from sharpening. But images, especially in some inexpensive compact
digital
cameras, are often oversharpened,
resulting in "halos" near edges. (Digital SLRs tend to be more
conservative.)
Oversharpened images look good in small enlargements straight out of
the
camera, but oversharpening creates artifacts that can be hard to get
rid
of. It's always best to sharpen late in the image editing process.

White balance
Digital cameras are used with light sources that have a wide range of color
temperature. This must be
addressed
when the RAW conversion is performed. Otherwise the image may look too
blue or yellow. Most cameras have several custom White Balance settings
in addition to an automatic setting that estimates the White Balance
based
on the spectral content of the image. Automatic White Balance
algorithms
work well most of the time, but they can fail for unusual
subjects,
for example, where a strong color dominates the scene. When you save an
image in RAW format and convert it later, you can make use of your calibrated
monitor to get White Balance just right.

Film scanning software
differs from RAW conversion software in only two respects. It doesn't
need
to perform a Bayer interpolation and it controls the motion of the
scanner.
Apart from that, all major functions-- gamma correction, tonal control,
sharpening, and white balance-- are identical. Most of the advice on
this
page applies equally to scanning.

The remainder of this page will show how performing RAW conversion
on
a computer, rather than in the camera, enables you to achieve optimum
tonal
quality.

Human
vision and tonal levels

The human eye is sensitive to relative
luminance differences. That's why we think of exposure in terms of zones
or f-stops,
where changing exposure by one f-stop or zone means halving or doubling
the light. The smallest luminance difference the eye can distinguish in
bright light (Delta L) is expressed by the Weber-Fechner
law,

In a photographic print, which has about a
100:1
luminance ratio, the eye can distinguish between 100 and 200 discrete
luminance
levels-- fewer than the 256 available in 8-bit B&W or 24-bit color.
Assuming 200 levels, this is an average of 30 discrete visible steps in
each of the 6.6 zones that comprise the 100:1 luminance ratio-- fewer
than
the 70 indicated by the Weber-Fechner law. The reason for the
discrepancy
is that the eye distinguishes fewer luminance levels in
the dark areas of a print due to visual interference-- mostly flare
light--
from the light areas. The number of luminance levels the eye can
distinguish decreases gradually from light to dark zones.

In a RAW, i.e., linear file, the pixel level
is
proportional to light intensity. An exposure change of one zone or
f-stop
involves halving or doubling the pixel level. Therefore, half the pixel
levels are in the top (brightest) zone, a quarter are in the second
zone,
a fourth are in the third zone, etc. A linear file with a bit depth of
8 (256 discrete levels) would contain 128 levels in the top zone, 64
levels
in the second, and so on. The top zone would have far more levels than
the eye can distinguish, while the lower zones would have relatively
few
levels-- only 8 and 4 in zones 5 and 6, respectively. "Banding" would
be visible in shadows. If much editing were applied to the image-- if
shadow
areas had to be lightened (dodged) or increased in contrast, the
banding
could become severe. In a color space with gamma = 2.2, the
distribution
of levels is much more uniform: there are 69 levels in zone 1 (close to
Weber-Fechner's magic number), 50 in zone 2, and so on. Zone 6 would
still
have 14 levels. Banding is invisible unless considerable editing is
applied.
Gamma = 2.2 may well be the ideal value for files with a bit depth of 8
(8-bit B&W and 24-bit color).

RAW (linear) and converted (gamma = 2.2)
levels
are shown in Table 1 for eleven zones of decreasing luminance, numbered
1 through 11. Columns 2-6 are for RAW (unconverted) data. The brightest
Normalized level in each zone, 1/2n-1 = {1, 0.5, 0.35, ...}
is shown in column 2 and the fraction of available levels, 1/2n
= {0.5, 0.25, 0.125, ...} is shown in column 3. The total number of
levels
for RAW files from 10, 12, and 14-bit A-to-D converters is shown in the
columns 4-6. Most current digital SLRs have 12-bit A-to-D converters.
The
rightmost five columns are for color spaces with gamma = 2.2. The
rightmost
two columns show the number of levels in each zone for 8 and 16-bit
depths,
representative of 24 and 48-bit color files.

Exposure

zone

RAW

RAWLevels
in each zone

Gamma = 2.2

Gamma
= 2.2Levels
in each zone

Normalizedlevel

Fraction

in
zone

10
bits

12
bits

14
bits

Normalizedpixel
level

8-bitlevel

Fraction

in
zone

8-bit

16-bit

1

1.0

1/2

512

2048

8192

1.0

255

0.27026

69

17712

2

0.5 (1/2)

1/4

256

1024

4096

0.72974

186

0.19722

50

12925

3

0.25
(1/4)

1/8

128

512

2048

0.53252

136

0.14366

37

9415

4

0.125
(1/8)

1/16

64

256

1024

0.38860

99

0.10502

27

6883

5

0.0625

1/32

32

128

512

0.28358

72

0.07664

20

5023

6

0.03125

1/64

16

64

256

0.20694

53

0.05593

14

3665

7

0.015625

1/128

8

32

128

0.15101

38

0.04081

10

2675

8

0.007812

1/256

4

16

64

0.11020

28

0.02978

8

1952

9

0.003906

1/512

2

8

32

0.08042

21

0.02174

6

1425

10

0.001953

1/1024

1

4

16

0.05868

15

0.01586

4

1039

11

0.000977

1/2048

--

2

8

0.04282

11

0.01157

3

758

A few things are evident from this chart.

Levels are lost when converting from a
12-bit
RAW
format (4096 levels total) to an 8-bit file (8-bit B&W or 24-bit
color).
But image quality in an 8/24-bit file will be adequate, though just
barely,
if the exposure is correct and little editing is required. This is
achievable
in studio environments, but less often when using "natural" (i.e.,
uncontrolled)
light.

16-bit files (16-bit B&W or 48-bit
color)
have
plenty of levels (65,536 total). You can edit to your heart's content
without
fear of banding or other artifacts arising from limitations of 8-bit
files.

When RAW conversion is performed within
the
camera,
i.e., when you save JPEG (or other standard) files instead of RAW
files,
you have little control over the process. The Canon EOS-10D has a
contrast
setting that gives a small amount of control, but it's awkward to
access
and it makes little difference.

If you save files as RAW (the sensor's
native
format)
and convert them later on a computer, you have enormous control over
tones
when you convert.

Dynamic
range

If we assume that the darkest useable zone has 8
levels (remember, the eye can distinguish fewer levels in shadows), the
chart tells us that a 12 bit A-to-D converter has a potential dynamic
(exposure)
range of 9 zones and a 14-bit A-to-D has a potential range of 11 zones.
Both these numbers are far in excess of the range ordinarily achieved
with
digital cameras. What is happening?

We can gain some insight by looking at a
simple
imaging system-- a camera/printer combination that reproduces images
with
no editing or manipulation. The two gamma equations in the section on RAW
conversion are applied exactly. The contrast of the print would be
identical to the scene contrast-- a nice idea in theory, but one that
rarely
produces "fine" prints because a print can only reproduce a tonal range
of about 100:1-- 6.6 f-stops. In this case the system dynamic range
would
also be 6.6 f-stops-- well under the camera's potential dynamic range.

The transfer curve (on a logarithmic scale) would look like
the straight
dashed blue line in Fig. 2. (Logarithmic scales correspond to human
vision
better than linear scales; they are discussed in detail below.) Camera
manufacturers are faced with a classic tradeoff: contrast
versus dynamic range. Most images look best with
enhanced
contrast, but if contrast is increased, dynamic range suffers. Dynamic
range can be increased by decreasing contrast, but images tend to look
flat. That won't sell cameras!

One way around this tradeoff is to apply a curve,
shown in solid black in Fig. 2. Contrast is reduced in the shadows and
highlights, but increased in the important midtone area, which is
responsible
for perceived contrast.

Figure 2.

I don't know whether curves are applied in the
RAW conversion algorithms used in cameras. There's no reason why they
shouldn't,
but if they are, camera manufacturers aren't saying. It would be part
of
the proprietary signal processing package that gives them a competitive
advantage. Camera manufacturers could do even more-- they could make
conversion
adaptive.
Low contrast scenes could have contrast expanded and contrasty scenes
could
have contrast compressed. Again, if any camera manufacturers are doing
this, they aren't telling. Perhaps it will be next year's big "advance."

In the meantime you can apply curves manually
with a flexible RAW conversion program like Capture
One DSLR, or in an image editor. Both approaches are described below.
One of the best uses of curves is to bring up dark levels that are lost
when RAW conversion is performed inside the camera, as described above.
This enables you to access your camera's hidden
dynamic range.

The number of A-to-D converter output levels isn't the only factor
that
limits dynamic range. Noise
is another, and it's extremely important. Unfortunately you won't find
much about it in camera spec sheets or glossy brochures. It's difficult
to quantify-- the amount of noise depends on the exposure and ISO
speed,
and it has a spectral distribution-- it's not a simple number. But it
has
a few important properties.

Noise is strongly correlated with pixel size: the larger the
pixel, the
lower the noise. Digital SLRs, which have 6.8 micron or larger pixels,
tend to have lower noise than compact digital cameras, which have 3.4
micron
or smaller pixels. Hence they have a larger hidden dynamic range. This
is one of the major advantages of DSLRs.

Signal-to-Noise Ratio (SNR)-- the amount
of
noise
relative to the signal, is worst in the darkest zones.

The higher the ISO speed, the higher the noise. Digital
cameras increase ISO speed by amplifying
the signal-- moving a given exposure to a lighter zone. For example, an
exposure that would fall in zone 9 (in Table 1) at ISO 100 would be
fall
zone 8 at ISO 200 and zone 7 at ISO 400, etc. Unfortunately, noise is
amplified
along with the signal, and if it's amplified enough-- if ISO speeds are
high enough, noise can become objectionable, particularly in dark
zones.
This can be particularly significant if you use curves to expand
dynamic
range.

To take maximum advantage of your camera's
hidden dynamic range you should set it to its lowest ISO speed.
High ISO speeds increase noise and make it difficult to expand dynamic
range through the use of curves.

Exposure

It may seem backwards to discuss exposure after
RAW
conversion, but it isn't. The optimum approach to setting exposure
depends
on the format you choose for storing the image; it's slightly different
for RAW than for standard image formats such as JPEG or TIFF. You
select
the storage format-- by intent or by default-- before
making the exposure.

If you choose to save the images in a
standard file
format, you should expose for tones that look
good-- as close as possible to the
tones you want in the final print. This works best if the scene dynamic
range is close to the dynamic range available in a print-- about 5 to 7
f-stops.

If you choose to save images in RAW
format,
you should
expose to capture maximum
information:
to maintain as much highlight and shadow detail as possible, even if
the
middle tones aren't what you want in the final print. You should strive
to capture all highlights except for bright light sources and specular
reflections. You should expose enough to capture detail in large shadow
areas. I mostly agree with the Luminous-Landscape.com
article, Expose
(to the) Right, which recommends setting the exposure to the
maximum
value that doesn't burn out highlights. (This applies only to
images
saved in RAW format.) However I wouldn't go too far. A little margin
doesn't
hurt; there are plenty of levels in 12-bit A-to-D converters. In
extreme
situations, you may want to make two
exposures and combine them.

Shadow and highlight detail are extremely
important
in fine full-toned prints. To my eyes, a print with dead shadows or
burnt
out highlights looks amateurish. One of the things that distinguished
the
glorious prints of Paul Strand, Edward Weston, and Ansel Adams is the
tonal
detail in shadows and highlights, as well as middle tones. If you
haven't
seen original prints by these great artists, it's time to make a
pilgrimage.

In either case, you should make use of the histogram--
the chart that shows the distribution of tonal levels. A histogram
should
be displayed in the camera's LCD monitor immediately after each
exposure.
Your camera's default settings may need to be changed: In the Canon
EOS-10D menu I set Review
to On (Info)
and Review time
to 8 sec.
I regularly make test exposures and check the histogram to be sure I
have
the correct exposure. If not, I reexpose using exposure compensation. I
delete test exposures immediately unless they happen to be "keepers."

Although I repeatedly stress the importance of
saving files in RAW format, there are instances where high quality can
be achieved using JPEGs. Claude Jodoin of Michigan uses JPEGs for
weddings
and other high volume events. RAW conversion is too slow for his
workflow.
He uses high quality lighting, an ExpoDisc
to set the White Point, and he knows how to expose properly, so images
look good straight out of the camera. The bottom line is satisfied
clients.
This doesn't work well for landscape photography, where lighting is
uncontrolled
and scene contrast varies all over the place.

Comparing
digital and film: gamma equals contrast.

The human eye, as we mentioned earlier, responds
to relative luminance differences. Relative differences are not
displayed uniformly when luminance is plotted on a linear scale, but
they
are on logarithmic
scales: relative differences such as doubling or halving the luminance
(changing it by one exposure zone) occupy the same distance,
independently
of the absolute level.

For this reason it makes sense to use
logarithmic
curves to display the relationships between exposure, film density,
pixel
level, and luminance. This is routinely done for film and papers, where
Film Density (-Log10(transmitted
light/incident light)) plays the same role as pixel level in digital
images.
Note that one unit on a Log10 scale (such as Density) equals
3.32 exposure zones (f-stops); one exposure zone equals 0.301 Density
units.

When the conversion curves in Fig. 1 are
replotted
on logarithmic scales, the characteristic curves become straight lines:
If y = xgamma, then log(y) = gamma * log(x).
Gamma is the slope
of the line; it is the change in the dependent variable (y-axis) for a
given change in the independent variable (x-axis). Using the RAW
conversion
plot in Fig. 3 as an example, gamma is the change in Log (pixel level)
for a given change in Log (exposure). In other words, gamma
is contrast.

Figure 3. Characteristic
curves for
the two
conversion processes in digital printmaking.

Both are displayed with logarithmic (base
10) scales.
In contrast to the linear display in Fig. 1, the transfer curves are
straight
lines.

Output luminance (print or
monitor) as a function of normalized file pixel levels.

The equivalence of gamma with contrast becomes
strikingly apparent when we compare Fig. 3 to film and paper
characteristic
curves in Fig. 4. The RAW conversion plot (Fig. 3, left) is similar to
the film curve (Fig. 4, left) and the print/monitor display curve (Fig
3, right) is similar to the photographic paper curves (Fig. 4, right).
Log (pixel level) is analogous to Density = -Log
(light transmission) in the film plot and Log exposure in the paper
plot.
Reflection density is the same as -Log
(luminance).
The average slope of the film and paper curves is called "gamma;" it is
indeed the same gamma used to characterize digital color spaces and
monitors.

The principal difference between the
digital and
conventional curves is in the "knee" regions near the response limits.
The knees tend to be more abrupt for digital.

Measuring
digital camera tonal response and dynamic range

With the new Imatest
program, digital camera tonal response and dynamic range to be
measured
easily and accurately using a transmission step wedge-- a piece of film
with zones of incresing density. The details of the measurement are in
the Imatest
Q-13
tour.

Here are the results for the Canon EOS-10D at ISO 400,
converted from
RAW format with Capture One LE. The upper left plot is the density
response curve. The lower left plot is noise measured in f-stops-- a
relative measure that corrresponds to the eye's response. Each step
represents 1/2 f-stop (a density step of 0.15).

The total dynamic range of the EOS-10D is 8.5
f-stops.
Dynamic range
improves slightly (but less than I expected) for 48-bit TIFF conversion
and ISO 100. Noise is significantly lower for 48-bit TIFF conversion.
The
shape of the response curve is a strong function of the conversion
software
settings. The curve on the right for Canon Zoom Browser with
Contrast
set to Low is very different from Capture One LE (though both are
half
an "S" curve), but the dynamic range hardly changes.

You can download
an evaluation
version of Imatest
that allows up to 20 runs of individual modules, and you can purchaseImatest
for a very affordable price.

"In
our opinion, we have not seen contemporary photographic prints on any
other
photographic paper, including all platinum and other "alternative"
papers,
that consistently rival the depth, luminosity, and tonal range of
photographs
printed on Azo paper."

Unless you plan to swap your DSLR for an 8x10,
take a careful look at the curve. It's different from the Polymax curve
in Fig. 4 and strikingly different from the digital camera curves in
Fig.
3.

Figure 5. Azo characteristic curve.

The key difference is in the knee
of the curve, indicated by the red arrow-- the highlight region where
density
starts increasing. The knee is much broader for Azo than for Polymax--
density increases more gradually. The knees in both film curves are in
stark contrast to the digital curves in Fig. 3, which cut off abruptly
at their limits. (The polarities of several parameters are reversed;
the
corresponding knee in Fig. 3, such as it is, is indicated by an arrow
in
the upper right.) Thanks to the knee, Azo holds highlight detail very
nicely.
It doesn't easily burn out. The knee gives Azo its superb luminosity.
(Luminosity
refers, of course, to highlights.)

The knee is not there by accident. It's the climax of an evolution
that
has spanned several generations of silver-based paper (and
photographers
as well). Digital printing has evolved enormously in the last decade,
but
it's still the new kid on the block. There are still some rough edges,
or perhaps... sharp corners.

Digital photographers need not despair.
The effects of the "knee" can be duplicated by Curve
controls using one of two
approaches.

Apply a curve control during RAW
conversion.
This
requires a high quality RAW conversion program like Capture
One DSLR. (I use C1
LE with my EOS-10D.) The crummy File Viewer Utility supplied free
with
Canon digital cameras won't do (though it would be a bargain at half
the
price).

Convert the RAW image to a file with
a bit
depth
of 16: 48-bit color or 16-bit B&W and apply the curves with an
image
editor. This allows you to get away using Canon's File Viewer Utility,
but you need an image editor that can support a bit depth of 16, like Picture
Window Pro (my favorite) or Adobe
Photoshop CS. Earlier versions of Photoshop had limited support for
16-bit editing and Elements doesn't support it at all. Results will be
inferior with a bit depth of 8.

Disclaimer
(for
Michael): The techniques presented below won't enable you to duplicate
Azo exactly; other factors such as tint and surface contribute
to
its unique appearance. But with practice, patience, and appropriate
equipment--
with a
well-calibrated monitor and
a high quality
printer-- you'll be able to
make beautiful luminous prints that meet the highest artistic standards.

And there's one more difference with digital.
You can fine tune the curve to match the needs of the image, precisely.
You aren't limited by the curve of the paper. (In my old B&W
darkroom
days I would to adjust the curve by mixing standard and low contrast
paper
developers-- Dektol and Selectol Soft. Digital curves are easier.)

1. Curve applied during RAW conversion

This image of Capitol Reef National Park isn't my favorite-- it could
use
a stronger foreground to anchor the composition, but neither Mother
Nature
nor the National Park Service cooperated. It's fine for illustrating
RAW
conversion using Capture
One LE.

Figure 6. Capture One LE Exposure tab.

Figure 7. C1 LE Levels tab with histogram

The histogram, visible when the Levels tab is selected, is shown in
Fig.
7. The levels tab allows you to set the minimum and maximum pixel
levels.
I moved the minimum from its default of 0 to 20 but didn't change the
maximum--
I used the Curve for that adjustment. To adjust the curve you need to
select
the Curve tab, as illustrated in Fig. 6. The curve I chose is quite
similar
to the Azo curve-- recall that polarities are reversed, so the lower
left
of the Azo curve corresponds to the upper right of the C1 LE curve.
There's
nothing rigid about the Azo curve; I choose whatever curve (or straight
line) works best with each individual image.

It's evident from this histogram, and from the Canon File
Viewer Utility
window in Fig. 8, that the issue with this image is not
the dynamic range. It's the low contrast of the original image-- the
result
of atmospheric haze.

Ed Harris uses a pre-CS version of Photoshop,
which forces him to do many operations in 24-bit color (bit depth = 8).
He often does considerable editing after RAW conversion. To get around
the 8-bit limitation, he does several RAW conversions, then combines
them.
You can achieve similar results (more easily, I believe) with the new
Stack
Images transformation in Picture Window
Pro 3.5.

Ed
Harris's
workflow for expanding tonal range

I
have developed a workflow for expanding the tonal range of a single
image
with Capture One. Some images just don’t make it using multiple
exposures
with blending. There may be ripples in a stream, leaves moving in the
wind,
or an animal in the scene.

I select
the image in C1, then crop and adjust it normally. The image's tonal
limitations
become obvious at this stage. Moving the exposure (EC) slider across
its
5-stop range reveals the possibilities of what can be done. (I
prefer Levels and Curves. C1 gives you several means of accomplishing
the
same end. --NK) I develop two, three,
or more versions at half or full stop EC increments, often applying
additional
enhancements to each version using C1's corrections tools. I don't
apply
sharpening at this stage (I use PK Sharpener Pro).

I move
each version onto a Photoshop layer. Using a soft brush I erase
incorrect
tonal values from each layer, revealing the correctly toned portion of
the layer below, thereby building a full valued image. The order of
layers
depends on the image and the complexity of the corrections. I typically
place the lightest image at the bottom of the stack and erase the
underexposed
portions of the darker layer above. I recommend working on just layer
pair
at a time. The whole array of Photoshop tools, particularly Masks, can
be applied to each layer pair. Flatten the image when you are satisfied
with the result.

I normally
develop 8-bit TIFF files for each of the versions because these will
all
become layers within Photoshop. Obviously 16-bit TIFF files can be
used,
but their value is limited because the file has to be converted to
8-bit
anyway. Photoshop CS will fix that!

This
technique is one of the great plusses of taking your pictures in RAW
mode.

2. Curve applied in the image editor

If you're stuck with a clunky RAW conversion program like the Canon
File
Viewer Utility, tonal control options are limited-- you can adjust
Digital
Exposure Compensation and Contrast (in 5 steps) as well as a few color
parameters, but there is no curves control. Preview image refresh is extremely
slow with the Canon FVU. For the most part you're stuck with the image
as captured. Here it is with the default camera settings. The quality
of
the preview image is--
pardon my French-- merde.

Figure 8. Canon File Viewer Utility

Despite the weaknesses in its interface, the Canon File Viewer
Utility
does a decent job in converting the RAW image. And if you convert to
48-bit
color you can edit with no loss of tonal levels. Fig. 9 illustrates how
to edit the image with the Color curves transformation in Picture
Window Pro.

The original image-- the anemic output of the
Canon File Viewer Utility-- is on the lower left. The preview image is
on the top. The Color Curves control box is on the right. I especially
like the way it combines the curves adjustment with a histogram-- it
gives
exceptional control. The curves behave differently from Capture One LE,
but the overall result is similar. I used the HSL
color representation, which tends to increase saturation as lightness
(L)
is decreased. I also boosted the Saturation (S) (not visible here).
There
are gaps in the lower (result) histogram because the preview uses
24-bit
color. The gaps aren't present in the histogram of the 48-bit
transformed
image (Fig. 10, right). But they are present when the same Color Curves
transformation is performed on an identical 24-bit color image (Fig.
10,
left) More information on using the Color Curves transformation can be
found in Digital Light and Color's
tutorial,
Using
Curves and Histograms (a 1.34 MB PDF download)..

The loss
of tonal
detail is obvious in the histogram
on the left, though it's probably not severe enough to cause serious
banding.
But if you apply multiple edits-- if you do much selective tonal
control
(dodging and burning), the degradations will accumulate. The tonal
gradation
of the final image won't be as smooth as it could be. And of course,
this
image, like most landscapes, can benefit from additional work. It’s by
no means complete at this stage.

Black
& white

All digital cameras and most photographic printers are
designed for
color: it takes effort and imagination to turn images into Black &
White. What a contrast to my youth, when color was almost impossibly
difficult
in the home darkroom! B&W is a more abstract medium; it's not as
closely
tied to reality. B&W prints typically require and tolerate more
manipulation--
dodging and burning. Read Ansel Adams' "The Print" if you're skeptical.
The results can be striking-- very different from color. And the tones
can be gorgeous.

To convert the image into B&W I used Picture
Window Pro's Monochrome transformation, which allows you to select
and preview a filter of any color. I describe its use in my page on Black
& White. I used a red-orange filter because pure red was a
little
harsh. I also did some tonal manipulation and contrast masking. In some
ways I prefer the B&W image.

Figure 11. B&W with red-orange filter

Newer printers such as the Epson 2200, which has both black and gray
ink cartridges, do an excellent job with B&W, though glossy
surfaces
still can't quite match the finest silver prints. (However, matte
surfaces
whip their silver counterparts.) They do even better if you use a piece
of software called a Raster Image Processor (RIP) to control the ink
mixture.
With an RIP you can minimize metamerism--
changes in the appearance of the print under different light sources,
which
are much more noticeable in B&W prints than in color.

Summary

Digital cameras can produce prints with
outstanding
tonal quality if you use the proper techniques for capture, storage,
and
editing.

If possible, store digital camera images
in
RAW format
and convert them on a computer.

If possible, use a RAW conversion program
that allows
a high degree control over image tones and color. A Curves control is
particularly
desirable.

Conversion to 48-bit color (with a bit
depth
of 16)
is strongly recommended. It's a must
if the RAW conversion program doesn't have flexible controls.

Take advantage of the ability of Curves
controls
to simulate the "knees" of traditional photographic papers-- especially
in the highlight regions.

This page focuses on tonal levels in
image
capture
and editing, but several equally important areas are discussed on other
pages: Your monitor should be well-calibrated
and you should have a high quality photographic
printer. If possible, use a color-managed
workflow with high quality profiles.

It takes practice, patience, and the right
equipment
to make fine photographic prints, but if you have the passion your
efforts
will be well rewarded.

IImages
and text copyright (C) 2000-2013 by Norman Koren. Norman Koren lives
in Boulder, Colorado, where he worked in developing magnetic recording
technology for high capacity data storage systems until 2001. Since 2003 most of his time has been devoted to the development of Imatest. He has been involved with photography since 1964.